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Abstract

Middle to Upper Turonian strata of the Cretaceous Western Interior Seaway record evidence of a large-scale sea level change. The Codell Sandstone Member of the Carlile Shale, exposed in the Pueblo, Colorado area, consists of two distinct facies that are interpreted to record this sea level change: an upward-coarsening unit interpreted as distal lower shoreface overlain by a heterolithic unit interpreted as estuarine. 87 Sr/86Sr ratio analyses show an isotopic excursion within the estuarine facies, which is attributed to freshwater input. The surface separating the two facies is interpreted as a sequence boundary and the base of a 10 meter thick incised valley fill. The time equivalent shoreline (highstand) for the lower shoreface strata of the Codell Sandstone Member is placed between central Kansas and central Missouri, 700 to 1100 km east of the study area. Brackish-water strata of the incised valley fill would require a minimum of 700 km lateral translation of the eastern margin of the Cretaceous Western Interior Seaway during deposition of the Codell Sandstone Member. This westward shift in the eastern shoreline resulted from a sea level fall that could have been as little as 30-60 meters.